Summary

We have performed a detailed comparison of our models with high quality data from a representative set of Galactic clusters. For the clusters with very well known ages, metallicities, and abundance ratios (M 67, 47 Tuc, and M 5), our models matched essentially all the line indices with a very high degree of consistency, for the right set of input parameters (even though for M 5 that was only possible once the contribution by blue horizontal branch stars to the integrated light of the cluster was removed). For NGC 6528, where the metal abundances are more uncertain and, in the case of carbon and nitrogen, unknown, we followed the procedure described in Section 4.4 in order to estimate the cluster age and metal abundances. The input parameters of the best-fitting model are well within the range allowed from previous work on stellar abundances and ages. In the process, we learned a few important lessons: 1) Outstanding consistency was reached for ages and iron abundances estimated on the basis of blue and red indices. We recall that one of the main goals of this modeling effort is to extend the accuracy and reliability of Lick index modeling into the blue, with an eye towards their application into distant galaxy work. The results of this Section positively qualify our models for such applications (for an initial effort, see Schiavon et al. 2006); 2) Use of C$_2$4668 in conjunction with the CN indices allows us to estimate carbon and nitrogen abundances reliably. While this is not surprising, previous attempts were hampered by difficulties in the modeling of these indices. Combining those indices with Ca4227 which is strongly influenced by contamination of the blue pseudo-continuum by CN lines, allows determination of calcium abundances; 3) Because Mg$_2$ can be strongly influenced by the contribution by stars in the lower main sequence, agreement between the magnesium abundances that are obtained from that index and Mg $b$ can only be achieved if the models include the correct input IMF; 4) Finally, the oxygen abundances are very important to help deciding what are the adequate theoretical isochrones used in the models. While this was known before, we showed that a substantial mismatch between the oxygen abundances of the input isochrones and that of the target stellar population can generate a small, but detectable, systematic effect in the ages that are inferred from the different Balmer lines.

We carry this newly acquired knowledge on to the next section, where we take a brief look at some galaxy data from the literature.